1. Field of the Invention
The present invention relates to a method and an apparatus for pulling a single crystal and, more particularly, a method and an apparatus for pulling a single crystal wherein a single crystal of silicon or the like is pulled by the Czochralski method (hereinafter, referred to as CZ method) and the like.
2. Description of the Relevant Art
As a silicon single crystal ingot used for manufacturing a substrate for forming a circuit component of a LSI (large scale integrated circuit) and the like, a silicon single crystal pulled by the CZ method has been mainly used. FIG. 3 is a diagrammatic sectional view of an apparatus for pulling a single crystal used for the CZ method, and in the figure, reference numeral 11 represents a crucible.
The crucible 11 comprises a bottomed cylindrical quartz crucible 11a and a bottomed cylindrical graphite crucible 11b fitted on the outer side of the quartz crucible 11a and supporting the quartz crucible 11a. The crucible 11 is supported with a support shaft 18 which rotates in the direction shown by the arrow in the figure at a prescribed speed. A cylindrical heater 12a of a resistance heating type and a perforated discoidal heater 12b are arranged around and below the crucible 11, respectively. A heat insulating mould 17 is concentrically arranged around the heater 12a. The crucible 11 is charged with a melt 13 of a material for forming a crystal which is melted by the heater 12a. On the central axis of the crucible 11, a pulling axis 14 made of a pulling rod or wire is suspended, and at the lower end part thereof, a seed crystal 15 is held by a seed chuck 14a. Each part mentioned above is arranged at a fixed place in a water cooled type chamber 19 wherein pressure and temperature can be controlled.
In pulling a single crystal 26, the pressure in the chamber 19 is reduced and kept in the reduced state for an appropriate period of time so as to sufficiently release a gas contained in the melt 13. Then an inert gas is introduced into the chamber from the upper part, so as to make an inert gas atmosphere under reduced pressure within the chamber. In the step of seeding, the seed crystal 15 held at the lower end of the pulling axis 14 is brought into contact with the surface of the melt 13, so as to slightly melt the front portion of the seed crystal 15 melt. In the next step of necking, while the pulling axis 14 is rotated on the same axis in the reverse direction of the support shaft 18 at a prescribed speed, a crystal (neck) having a diameter narrowed down to 2-4 mm and a length of 20-40 mm is made to grow from the lower end portion of the seed crystal 15. After the necking step, the diameter of the single crystal 26 is gradually increased to form a shoulder having a prescribed diameter, and then, a main body is pulled which forms to a product.
In the last stage of the pulling, in order to prevent inducement of high density dislocation from being caused by a steep temperature gradient, the diameter of the single crystal 26 is decreased, leading to the formation of an end-cone and a tail end, and then, the single crystal 26 is separated from the melt 13. Cooling the single crystal 26 is at the end of the pulling. As described above, pulling a single crystal 26 comprises a great number of steps, and in general, it takes several tens of hours from melting a material for a crystal to the completion of pulling a single crystal 26.
Recently, in order to achieve more efficient production of a single crystal ingot and improvement in the yield of chips, the diameter of a pulled single crystal ingot has gradually become larger and the quantity of material for a single crystal required at one pulling has also become larger. Accordingly, the diameter of the crucible to make the material for a single crystal melt therein has also become larger, the required power of a heater has also become higher, and the time required for pulling a single crystal has also become longer. Consequently, a quartz crucible in direct contact with the melt has been exposed to a high temperature for a long period of time, resulting in more and more serious deterioration thereof. The deterioration of the quartz crucible allows bubbles entrained therein to grow, and when the bubbles are released, the melt is contaminated with quartz particles. When the quartz particles become incorporated into a single crystal at the crystal growth interface, the occurrence rate of dislocation increases.